PROJECT SUMMARY/ABSTRACT: Large-scale genomic studies of human tumors have uncovered a series of characteristic ?mutation signatures?; i.e., statistically enriched patterns of DNA substitutions and rearrangements common across tumors. While some of these signatures are associated with particular cancer subtypes, known genotoxic insults, and/or rough mutational patterns arising from specific DNA repair defects, most remain uncharacterized without known biological correlates. To determine the mechanisms by which mutation signatures arise, we will initially focus on triple-negative breast cancers (TNBCs) with lesions in the BRCA1 tumor suppressor pathway. Recent studies have identified at least five distinct mutation signatures in TNBCs. However, we do not know if additional signatures exist, nor do we understand the molecular mechanisms by which these signatures are generated. BRCA1 acts to preserve genome integrity through multiple processes, including homology-directed repair (HDR) and stalled fork protection (SFP), and it does so in association with a number of distinct protein partners. Thus, the mutation signatures associated with TNBC may reflect the loss of particular BRCA1 pathway functions and/or defects in other as yet unidentified DNA repair factors. To address these issues, we will combine expertise in computation analysis of genomic data and the molecular biology of DNA repair to 1) elucidate the molecular mechanisms responsible for TNBC mutation signatures, and 2) determine the full spectrum of mutation signatures associated with TNBC patients bearing pathogenic lesions in the BRCA1 pathway.